Restricting a class to a certain amount of objects in Java

Question

I would like to ask if there is a way to restrict a certain class to make more than a certain amount of instances. And if it is possible to make the compiler ("Eclipse") underline that line, when you try to make another instance (like it is an error in the code), or something along those lines?

Just to clarify for those who would say this is a bad idea, I am making a chess game, so I need it to not be able to make more than certain amount of playing pieces.

JB Nizet · Accepted Answer · 2016-03-07T20:46:11.520

Eclipse can't do that at compile time, because it can't tell how many times a piece of code will be executed, and thus how many instances are created.

But you can design your class to only create a given number of instances, and make it impossibale to create more. For example:

public class Limited {
    public static final List<Limited> ALL_INSTANCES = 
        Collections.unmodifiableList(createInstances());

    private int id;

    private static List<Limited> createInstances() {
        List<Limited> result = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            result.add(new Limited(i));
        }
    }

    private Limited(int id) {
        this.id = id;
    }
}

Since the constructor is private, the only 10 instances available are the ones in ALL_INSTANCES.

That said, that is not necessarily a good idea. Let's say you're creating a chess game. So by your logic, you shouldn't be able to create more than 2 King instances. What if your app handles 10 games at a time? Do you really want at most 2 Kings, or do you just want each ChessGame instance to have a black and a white King? Maybe all you need is something like

public class ChessGame {
    private King blackKing = new King(BLACK);
    private King whiteKing = new King(WHITE);
    ...
}

Thanks, just so I'm sure I got it right, I am making the objects in the class itself, and I cannot make objects outside of it due to the private constructor? — Poyr23, Mar 07 '16 at 20:44
Yes, but you should read the addendum of my answer. Most of the time, doing that is not a good idea. — JB Nizet, Mar 07 '16 at 20:46

score 0 · Answer 2 · answered Mar 07 '16 at 20:38

You can simply create the required number of objects - say by instantiating your class within a loop and then set a flag which is checked by the constructor so as to throw exception when an attempt is made to create one more. You can make the constructor private and have a static method to generate your instances.

Restricting a class to a certain amount of objects in Java

2 Answers2